As former President George W. Bush, joined by President Obama and three living former presidents, recently dedicated his library this week in Dallas, it's important to remember that presidential libraries are relatively new. In 1941, while he was still in office, Franklin D. Roosevelt established the first such archive in Hyde Park, N.Y., to preserve personal papers and mementos from his time in office. His successor, Harry Truman, signed the Presidential Libraries Act into law, authorizing the National Archives to help set up and operate these treasure troves of American politics and policy.

There are now 13 presidential libraries. From the beginning, these institutions have been grand storehouses of history. But in recent years, they've also become home bases for former presidents' efforts to make the world better.

That's the context for the George W. Bush Presidential Center. It will be an archive and a museum, certainly, but it will also be a hub for the former president and wife Laura to pursue their long-standing passions, such as fighting disease in Africa and inspiring dissidents in Iran, Burma and Cuba.

A big reason the libraries have evolved in recent years into hubs of social action is that so many presidents today leave office with productive decades still ahead of them. With the exception of Ronald Reagan, who left the White House two weeks short of age 78, modern U.S. presidents have been reentering civilian life at around the average age of Fortune 500 CEOs.

That leaves these dynamic men facing a unique challenge: What to do after leaving the most powerful job on the planet?

Liberated from the bonds of office, the paths chosen by former presidents have tended to mirror the public service causes that drove them to run for office in the first place.

Lyndon Johnson, Richard Nixon and Gerald Ford were all active in public life following their presidencies, writing books, speaking out on politics and establishing presidential libraries. But the man who really invented the modern post-presidency was Jimmy Carter, who left office at age 56 and quickly established the Carter Center, in Atlanta, which has focused on curbing neglected tropical diseases and observing elections in tough environments. Thanks in large part to the center's efforts, cases of Guinea worm disease have dropped from 3.5 million in 1986 to fewer than 542 worldwide today.

One of the surprises of George H.W. Bush's rich post-presidential life was his joining forces with Bill Clinton, the Democrat who defeated him in 1992. Bush, who left office at 68, and Clinton, who left at age 54, have teamed up to raise funds for emergency aid in the wake of major global disasters, including the 2004 Indian Ocean tsunami and hurricanes Katrina and Rita.

Clinton's foundation has focused primarily on reducing economic inequality and improving global health, with a focus on HIV/AIDS, malaria and tuberculosis. The Clinton Global Initiative, meanwhile, convenes leaders to make hundreds of specific commitments to improve lives.

George W. Bush left the White House four years ago, a vigorous 62. Since then, away from cameras and partisanship, he too has pursued causes that have motivated him since before he ran for governor of Texas.

Those causes have formed the agenda for the George W. Bush Institute, which was launched in November 2009 and now quietly manages 13 programs linked by a common thread of promoting freedom by removing constraints on realizing human potential.

As Bush has said, "One aspect of freedom is for people to be free from disease." For that reason, his institute has focused on disease, particularly on cervical cancer, which needlessly kills hundreds of thousands of women in Africa. The Bush Institute is working with partners that include the State Department and private foundations and businesses to diagnose and treat the disease in early stages in such countries as Zambia and Botswana.

Another Bush Institute program has brought dozens of talented Egyptian women to the U.S. to provide them with the tools to free them to build strong civil society institutions at home. Other programs seek to improve the quality of America's school principals and to intervene in middle school to prevent dropouts later.

Obama will be 55 when he leaves office. Many of the presidential hopefuls for the 2016 election are in their 40s and 50s, and would likewise leave office with many fruitful years ahead. As new members join this elite club, we will be able to look forward to some fascinating post-presidential careers.

James K. Glassman is the founding executive director of the George W. Bush Institute.